1. Field of the Invention
The present invention generally relates to memory devices. More specifically, the present invention relates to magnetoresistive random access memory (MRAM) devices.
2. Description of the Related Art
A MRAM device comprises a plurality of memory cells that store information in the form of a direction of magnetization of a ferromagnetic region. The memory cells are organized in a two-dimensional array wherein each memory cell is disposed at a crossover point between two intersecting conductive lines. Such lines are commonly referred to as bit/word lines or, individually, as a bit line and a word line. During the operation, the bit/word lines facilitate bi-directional transmission of information within the MRAM device.
The bit and word lines are used to select an individual memory cell and to enter (i.e., xe2x80x9cwritexe2x80x9d) an information content into the memory cell or evaluate (i.e., xe2x80x9creadxe2x80x9d) the information content that has been stored in the cell. Such lines form a network that comprises xe2x80x9cglobalxe2x80x9d and xe2x80x9clocalxe2x80x9d bit and word lines. To facilitate the writing and reading operations, the word and bit lines use controlled current sources and controlled current switches.
During a writing operation, the current sources provide pulses of a current (i.e., xe2x80x9cwrite currentxe2x80x9d) of pre-determined value that propagate in a specified direction along the bit and word lines. The write currents enter the information content (i.e., xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) in the selected memory cell. Such currents do not pass through the memory cells.
During a reading operation, a sensing circuit of the MRAM device determines a value of electrical resistance of a selected memory cell. The resistance of the memory cell is different based on whether the xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d has been stored in such a cell during the writing operation. The read currents may propagate through the memory cell and portions of the bit and the word lines coupled to the memory cell.
In MRAM devices having high-density arrays of the memory cells, the lengths of the bit/word lines tend to increase. Long bit/word lines have high electrical resistance and, in operation, require high-power current switches. Such current switches occupy large areas on a MRAM chip and, as such, decrease the information capacity of the chip (measured as a number of the memory cells per unit area of the chip), as well as increase the costs of fabricating the MRAM devices.
Therefore, there is a need in the art for an improved circuit configuration of a current switch for a bit line or a word line of a magnetoresistive random access memory (MRAM) device.
A circuit configuration for a bi-directional current switch of a bit line or a word line of a magnetoresistive random access memory (MRAM) device is disclosed. The current switch comprises a bi-directional switch and a voltage driver that reduces the ON resistance of the bi-directional switch. In one embodiment, the bi-directional switch comprises a line transistor (e.g., MOSFET transistor), and the voltage driver comprises a pulse generator coupled to a transistor switch that controls a gate of the line transistor. In operation, the voltage driver facilitates a high potential of a gate of the line transistor.
Another aspect of the invention is a circuit configuration for writing an information content in a memory cell of a MRAM device, comprising a bi-directional bit line or a bi-directional word line coupled to a memory cell of a plurality of such cells, two driving circuits, each driving circuit comprising the inventive bi-directional current switch and a sink transistor, and a controller controlling the driving circuits.
Yet another aspect of the invention is a circuit configuration for a unidirectional current switch of a MRAM device. In one embodiment, the current switch comprises a line transistor and a voltage driver, wherein the voltage driver comprises a pulse generator coupled to a transistor switch controlling a gate of the line transistor. In operation, the voltage driver facilitates a high potential of a gate of the line transistor.
Still another aspect of the invention is a circuit configuration for writing an information content in a memory cell of a MRAM device, comprising a unidirectional bit line or a unidirectional word line coupled to a memory cell of a plurality of such cells, a driving circuit comprising the inventive unidirectional current switch and a sink transistor, and a controller controlling the driving circuit.